A typical coating finish over a substrate comprises some or all of the following layers: (1) one or more primer layers that provide adhesion and basic protection, and also cover minor surface unevenness of the substrate; (2) one or more colored layers, typically pigmented, that provide most of the protection, durability and color; and (3) one or more clearcoat layers that provide additional durability and improved appearance. A colored topcoat layer can be used in place of the colored layer and clearcoat layer.
In some industrial applications, such as coating metal pipes, trucks, large industrial equipments, and large entertainment equipments, it is often desired to complete the coating process in a short period of time while still achieving good adhesion, protection, durability and appearance. Conventional coating compositions for colored layers are often less optimized for good adhesion directly to metals. In addition, a conventional coating composition typically produces a thin cured dry coating layer that may not have sufficient thickness to cover unevenness of the substrate if only a single layer is used. That could result in undesired appearance. When conventional coatings are applied at a high coating thickness, surface coating defects such as microfoaming, low gloss, and sagging may occur. Thick coating layers are typically prone to sagging defects, especially for coating layers applied on vertical surfaces.
Therefore, there is a need for a coating composition that can produce a coating being sag-free at high coating thicknesses, having good direct-to-metal adhesion, and good appearance.